The X-ray beam in most computer tomography (CT) scanners is generally polychromatic. However, third-generation CT scanners generate images based upon data according to the energy integration nature of the detectors. These conventional detectors are called energy-integrating detectors and acquire energy-integrated X-ray data. On the other hand, photon-counting detectors are configured to acquire the spectral nature of the X-ray source, rather than the energy-integrated nature. To obtain the spectral nature of the transmitted X-ray data, the photon-counting detectors split the X-ray beam into its component energies or spectrum bins, and count a number of photons in each of the bins. The use of the spectral nature of the X-ray source in CT is often referred to as spectral CT. Since spectral CT involves the detection of the transmitted X-ray at two or more energy levels, spectral CT generally includes dual-energy CT by definition.
Photon-counting detectors in computed tomography imaging systems are often produced from semiconductor materials, such as Cadmium Zinc Telluride (CdZnTe), often referred to as CZT, Cadmium Telluride (CdTe), and Silicon (Si), among others. Traditional PTF direct-conversion photon-counting detectors (CdZnTe or CdTe) suffer from severe energy resolution loss in order to reduce charge time-of-flight. Further, pixelated non-PTF direct-conversion PCDs have long time-of-flight due to longer charge collection time, which limits the counting performance. Furthermore, polar effects and K-escape, which is caused by partial transport of the primary energy, e.g. X-ray energy, through another quantum, e.g. an X-ray quantum, to a neighboring pixel also degrade the performance in the detector.